Heartworm infection is caused by a filarial organism, Dirofilaria immitis. At least 70 species of mosquitoes can serve as intermediate hosts; Aedes, Anopheles and Culex are the most common genera acting as vectors. Patent infections are possible in numerous wild and companion animal species. Wild animal reservoirs include wolves, coyotes, foxes, California gray seals, sea lions, and raccoons. In companion animals, heartworm infection is diagnosed primarily in dogs and less commonly in cats and ferrets. Heartworm disease has been reported in most countries with temperate, semitropical, or tropical climates, including the USA, Canada, Australia, Latin America, and southern Europe. In companion animals, infection risk is greatest in dogs and cats housed outdoors, but any dog or cat, indoor or outdoor, is capable of being infected.
Mosquito vector species acquire microfilaria (a neonatal larval stage) while feeding on an infected host. Once ingested by the mosquito, development of microfilariae into the first larval stage (L1) occurs. They then actively molt into the second larval stage (L2) and again to the infective third stage (L3) within the mosquito in 1 to 4 weeks, depending on environmental temperatures. When mature, the infective larvae migrate to the labium of the mosquito. As the mosquito feeds, the infective larvae erupt through the tip of the labium with a small amount of hemolymph onto the host's skin. The larvae migrate into the bite wound, beginning the mammalian portion of their life cycle. In canids and other susceptible hosts, infective larvae (L3) molt into a fourth stage (L4) in 3 to 12 days. After remaining in the subcutaneous tissue, abdomen, and thorax for about 2 months, the L4 larvae undergo their final molt at day 50 to 70 into young adults, arriving in the heart and pulmonary arteries about 70 to 120 days following initial infection.
The only available heartworm adulticide is melarsomine dihydrochloride, which is effective against mature (adult) and immature heartworms of both genders. Heartworm infection is preventable with macrolide prophylaxis. Year-round prevention is advised because of the potential for severe consequences, regardless of the housing status of the animals. Formulations of the macrolide preventives ivermectin, milbemycin oxime, moxidectin, and selamectin are safe and effective as prescribed for all breeds of dogs. Ivermectin/pyrantel pamoate (hookworms and roundworms) and milbemycin (hookworms, roundworms, and whipworms) also provide control of intestinal nematodes. At the approved dose, milbemycin kills microfilariae quickly, and in the face of high microfilarial concentrations a shock reaction may occur. Thus, milbemycin should not be administered without close monitoring as a preventive in dogs with high numbers of microfilariae. Ivermectin for cats is safe and effective at 24 μg/kg, PO, once monthly. Formulations of selamectin and a combination of imidacloprid/moxidectin are labeled for both dogs and cats.
Inter alia due to the possibility of resistance against existing drugs, there is a continuous need for finding new drugs that are active against Dirofilaria immitis (which includes any of the non-adult animal stages of the organism), which drugs can be used to treat an infection therewith (which treatment may by to prevent the infection or to therapeutically reduce the infection).